The Institute of Electrical and Electronics Engineers (IEEE) has developed a standard for supplying power to an Ethernet device over Ethernet cabling. The standard is labeled “IEEE 802.3 clause 33” (hereinafter “the POE Standard”). The POE Standard defines a Power over Ethernet (POE) system as a system that delivers power over unshielded twisted-pair wiring from a Power Source Equipment (PSE) device to a Powered Device (PD) located at opposite sides of an Ethernet link. Such devices previously required two connections: a network cable connection and a power supply connection. However, the POE system provides a means for supplying power and data to a network device through the same Ethernet cable.
The term “PSE” refers to a device that applies power to the Ethernet cable, which may be used to deliver both power and data to a PD. A PSE device can be an Ethernet switch, router, other network switching equipment, or midspan device of a data communications network. The term “PD” refers to a device that is connected to the link and that is configured to draw power and/or request power from the PSE device through the link. PDs can include a wide variety of devices, including digital Internet Protocol (IP) telephones, wireless network access points, personal digital assistants (PDAs), notebook computer docking stations, cell phone chargers, Web cameras, process control devices, HVAC thermostats, and other electronic devices.
Conventionally, a PSE device may include a plurality of network ports, which are configurable to connect to multiple PDs and to other Ethernet devices that are not compatible with the POE Standard. Accordingly, the POE Standard specifies a POE Standard PD detection process that is executed by a PSE device to detect a PD connected to the network port. In particular, the PSE device applies a signal to the network port, and the PD participates in the detection procedure by presenting a PD signature defined by the POE Standard, namely a signature resistance in a range from 19 to 26.5 kΩ. While the POE Standard specifies the electrical characteristics of the PD signature and defines limits on the voltage (between approximately 2.8V and 10V) and the current (less than approximately 5 mA) for use in the detection process, the POE Standard does not specify any particular technique for detecting the resistance.
One common technique for detecting the PD signature includes applying a small current to the network port, typically in a range of about 150 μA to 400 μA, and measuring a voltage at the network port. When the PSE device applies such a current to a network port connected to a PD that complies with the POE Standard, the measured voltage falls within the range of approximately 2.8V to 10V.
Another technique involves applying a voltage within the range of 2.8V to 10V to the network port, and measuring an associated line current. The applied voltage can be divided by the line current to detect the resistive PD signature.
However, some PDs, which were manufactured before the adoption of the POE Standard, do not provide the expected PD signature in response to the PSE's detection signal. Such PDs are referred to as legacy PDs. Many of these legacy PDs used entirely different methods of detection. In one instance, the receive (RX) pins of the legacy PD were connected to the transmit (TX) pins by a relay or other electronic circuit. The legacy PSE device would probe for such a legacy PD by sending a modified fast-link-pulse and monitoring the return signal. If the legacy PD was detected based on the return signal, the PSE would provide power via the Ethernet cable, causing the relay or other circuit to disconnected the RX and TX pins and allowing the Ethernet link to operate normally. However, since not all of the legacy PDs can support the modified fast-link-pulse PD detection, supporting fast-link-pulse PD detection alone may be insufficient to detect such legacy PDs.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items.